Expendable Pattern casting, also known as lost foam casting, is a known casting technique in which a pattern formed of an polymeric foam material, such as polystyrene or polymethylmethacrylate, is supported in a flask and surrounded by an unbonded particulate material, such as silica sand. When the molten metal contacts the pattern, the foam material decomposes with the products of decomposition passing into the interstices of the sand while the molten metal replaces the void formed by the expended foam material to produce a cast part which is identical in configuration to the pattern.
In the conventional expendable pattern casting process, the sand which surrounds the pattern and fills the cavities in the pattern is unbonded and free flowing and this differs from traditional sand casting processes, wherein the sand is utilized with various types of binders. However, after compaction, the unbonded sand density is generally higher than the density of molds made with bonded sand, and therefore the rigidity or stiffness of compacted unbonded sand is not deficient relative to bonded sand molds. Traditionally, silica sand has been used exclusively as the molding material in expendable pattern casting because it is readily available and inexpensive.
It has been recognized that a conventional expendable pattern casting process is only capable of matching the precision of green sand casting and has not been considered a precision sand casting process. This lack of precision for a process that uses metal molds to make the foam patterns, has been a drawback of the process.
In cast cylinder blocks for internal combustion engines, the axes of the cylinder bores must be maintained within a specific tolerance. After casting the cylinder bores are simultaneously machined by automated machining equipment. If the axes of the cylinder bores are not within the specified tolerance, the bores cannot be satisfactorily machined, with the result that the engine block must be scrapped.
In casting an engine block using an expendable foam casting process, the foam pattern contains a number of cylindrical bores or cavities and in the casting process, the bores are filled with the unbonded sand. The shrinkage of the molten metal on solidification can be accurately calculated, and thus the diameters of the cylindrical bores in the pattern are increased to reflect the shrinkage of the metal. However, if the sand contained within the bores does not accommodate the shrinkage of the molten metal and resists this shrinkage, an unpredictable metal shrinkage is obtained which causes a lack of precision in the cylinders of the cast engine block.
One skilled in the metal casting art does not expect the temperature of the sand to have a significant influence on the dimensional size of castings produced by any of the sand casting processes. The major reason for this oversight is because, except for the expendable pattern casting process which uses unbonded sand, sand casting processes employ bonded sand molds that are used at the semi-uncontrolled ambient temperatures seen on the foundry floor. The economics of achieving through-put in the foundry and the cost of carrying an unnecessarily high inventory of molds on the foundry floor, dictate that the bonded sand molds be used in some orderly just-in-time approach. As a result, it is not the practice of foundries to heat or to cool the sand molds in a separate "conditioning" or stabilizing area and there has been no recognition that the temperature of the sand mold has a significant effect on the dimensional size or tolerances of the resulting castings that are produced with the molds.